Necropsy results were inconclusive about the cause of death and the zoo is waiting on pathology reports based on tissue samples, zoo spokesman Hova Najarian said, KGW-TV reports.

Because they were new arrivals, the animals had been in quarantine under the care of the zoo's veterinary staff.

The remaining three monkeys, including a 5-week-old baby, appear to be healthy.

The deaths come months after an orangutan died in Oregon Zoo's care. An investigation found mistakes in the orangutan's treatment and led to the firing of the zoo director and a senior veterinarian.

The deaths of the cotton-top monkeys appear to be less related to the zoo's veterinary practices than the orangutan's death, due to the fact that the monkeys died shortly after arriving, according to KGW-TV.

The zoo did not disclose to KGW-TV which facility the monkeys came from.